Intraluminal stenting is useful in treating tubular vessels in the body that are narrowed or blocked and it is an alternative to surgical procedures that intend to bypass such an occlusion. When used in endovascular applications, the procedure involves inserting a prosthesis into an artery and expanding it to prevent collapse of the vessel wall.
Percutaneous transluminal angioplasty (PTCA) is used to open coronary arteries, which have been occluded by a build-up of cholesterol fats or atherosclerotic plaque. Typically, a guide catheter is inserted into a major artery in the groin and is passed to the heart, providing a conduit to the ostia of the coronary arteries from outside the body. A balloon catheter and guidewire are advanced through the guiding catheter and steered through the coronary vasculature to the site of therapy. The balloon at the distal end of the catheter is inflated, causing the site of the stenosis to widen. Dilation of the occlusion, however, can form flaps, fissures or dissections, which may threaten re-closure of the dilated vessel. Implantation of a stent can provide support for such flaps and dissections and thereby prevent reclosure of the vessel. Reducing the possibility of restenosis after angioplasty reduces the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary.
A stent is typically a hollow, generally cylindrical device formed from wire(s) or a tube and the stent is commonly intended to act as a permanent prosthesis. A stent is deployed in a body lumen from a radially contracted configuration into a radially expanded configuration, which allows it to contact and support the vessel wall. The stent can be made to be either radially self-expanding or expandable by the use of an expansion device. The self-expanding stent is made from a resilient material while the device-expandable stent is made from a material that is plastically deformable.
A plastically deformable stent can be implanted during an angioplasty procedure by using a balloon catheter bearing the compressed stent, which has been loaded onto the balloon. The stent radially expands as the balloon is inflated, forcing the stent into contact with the body lumen, thereby forming a support for the vessel wall. Deployment is effected after the stent has been introduced percutaneously, transported transluminally and positioned at a desired location by means of the balloon catheter. A balloon of appropriate size and pressure may be first used to open the lesion. The process can be repeated with a stent loaded onto a balloon. A direct stenting procedure involves simultaneously performing angioplasty and stent implantation using a stent mounted on a dilatation balloon. After the balloon is withdrawn, the stent remains as a scaffold for the injured vessel.
In particular, the present invention relates to stents which can be delivered to a body lumen and which can be deployed at a treatment site by expanding the stent radially from a crimped state into an expanded state in which the stent supports the walls of the vessel at the treatment site. As noted above, the radial expansion is achieved by inflating a balloon on which the stent is located. One problem that can arise with this type of stent delivery system is that the stent may accidentally be displaced on the balloon as the delivery system negotiates torturous body vessels along its path to the treatment site. In order to ensure proper placement of the stent at the treatment site, one must avoid relative movement between the stent and the balloon. One means by which this risk of relative movement between the balloon and stent may be lessened is to form pillows on the balloon on either side of the stent to help prevent the stent from slipping off the balloon. Another means of achieving this object is to securely mount the stent onto the balloon.
An existing process for securely mounting the stent uses a series of tetrafluoroethylene (TFE) sheaths positioned over the stent/balloon assembly followed by a heat set operation using hot air and pressure applied to the balloon. The steps involved in this prior art process are as follows. First, the stent is crimped down to the required size. TFE sheaths to be used in the process are cut to length and slits are made in the sheaths to facilitate their removal when the process is complete. First, second and third TFE sheaths 20, 21 and 22 are assembled as shown in FIG. 1 (prior art). This assembly is then loaded onto the delivery system. The stent 23 is loaded onto the delivery system and positioned on the balloon 24 between the markers 25 and 26. The TFE assembly is then positioned carefully over the stent/balloon assembly. This positioning defines the proximal balloon pillow 27. A fourth TFE sheath 28 is then loaded onto the distal end of the assembly and positioned relative to the distal end of the stent. The positioning of this sheath defines the distal balloon pillow 29. The complete assembly is shown in FIG. 2 (prior art). This assembly with the delivery system, stent and TFE sheaths is then loaded into a heat set machine. This machine applies a high pressure to the balloon interior through the delivery system luer fitting so as to push the balloon against the ID (internal diameter) of the stein. The balloon is prevented, by the TFE sheaths, from fully inflating. This assembly, under pressure, is moved into a hot air source so that heating is applied to the stent/balloon assembly. This combination of heat and pressure secures the stent to the balloon. After the heat fix operation, the TFE sheaths are removed from the assembly and discarded. Thus, the process of the prior art is time consuming, wasteful of materials and can be used to manufacture only limited range of design of the final profile.
The present invention seeks to alleviate the disadvantages of the prior art method. The method of the present invention uses a mould in place of the TFE sheaths. This mould is part of a new heat set apparatus, also provided by the present invention.